1. Field of the Invention
This invention relates to moving storage apparatus of the type in which the read/write transducer is in contact with the storage medium when at rest and "flies" above the medium surface when the medium is moving at operating speed, and more particularly to a method and apparatus for measuring the flying height of the transducer over the storage medium.
2. Description of the Prior Art
In high speed data processing systems, magnetic disks have been employed for large storage capacity requirements. Data is read from and written onto the disks by magnetic transducers commonly called magnetic heads which are positioned over the disk during the retrieval and storage of data on tracks of the disks. The requirements for higher data density on magnetic disks have imposed a requirement to read and write more data on narrower tracks located on the disk. The achievement of a higher data density requires increasingly narrow transducing gaps and increasingly less spacing or clearance commonly called flying height, between the magnetic transducer and the disk recording surface. It becomes increasingly more difficult to maintain this low flying height constant to the degree required to reliably record and read data at the higher data density.
Prior art methods for measuring flying height have included various capacitive and optical techniques which require special "test" disks or sliders. These methods are unable to measure the slider/disk clearance in-situ or in a direct manner. The clearance between real sliders and disks is inferred from the measurements from the "test" sliders or disks. This method has been suitable up to the present, but, with the low flying heights now required, the accuracy and precision of the prior art indirect measurements are no longer satisfactory.
The effect of head/media spacing on the amplitude of magnetic read back signals is described by R. L. Wallace, Jr. in "The Reproduction of Magnetically Recorded Signals", The Bell System Technical Journal, Vol. 30, Oct. 1951, pp. 1145-1173. This publication includes the Wallace equation which expresses the dependence of the readback voltage on various recording parameters including the head/disk spacing.
Using the modulation of the read signal envelope to measure variations in head/disk spacing is described in "Head Flight Height Monitoring" by Boudet et al in the IBM Technical Disclosure Bulletin, Vol. 11, No. 12, May, 1969, p. 1650. The control factor described there is based on a comparison of the currently detected read signal level with the average of previously detected levels.
The modulation of the read signal envelope to measure variations in head/disk spacing is also described in Shi et al, "Use of Readback Signal Modulation to Measure Head/Disk Spacing Variations in Magnetic Disk Files", Technical Report No. 11, The Center for Magnetic Recording Research, University of Calif., San Diego, Dec., 1985. The readback signal modulation technique is used in conjunction with a laser-doppler vibrometer to measure simultaneously the spacing variation and disk vibrations that result from various laboratory induced impacts directly on the disk of an operating disk file. While this apparatus is useful as a laboratory tool in the research and development of magnetic disk files, the disk file structure must be altered to accommodate the laser-doppler vibrometer and the equipment required is very costly.
"Effect of Flying Height Variation on Offtrack Data Handling" by Morris et al, IEEE Trans. on Magnetics MAG-17 No. 4, Jul., 1981, pp. 1372-1375 infers flying height variations by modulation of the magnetic head read signal and correlates the flying height variation to offtrack data handling capability.
The prior art has not shown a method and apparatus for measuring the spacing between a magnetic transducer and the recording medium in-situ in a direct manner in an operational magnetic disk storage system.